ksmbd: fix use-after-free in smb_break_all_levII_oplock()

There is a room in smb_break_all_levII_oplock that can cause racy issues
when unlocking in the middle of the loop. This patch use read lock
to protect whole loop.

Cc: stable@vger.kernel.org
Reported-by: Norbert Szetei <norbert@doyensec.com>
Tested-by: Norbert Szetei <norbert@doyensec.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>